So you might agree on having a set of bugs fixed, for instance, or agree on getting a piece of functionality implemented. I'm not an expert on ROWE, but as I understand it, the key is that you agree on results which you (as a manager) are satisfied with and which the team (who will produce those results) is satisfied with.
Then, you don't care about how many hours they work. At the agreed time, you expect the result that you agreed on. If it's there, no problem, move on to the next bit of work that needs to happen. If it's not there, then you can sit down with your team and figure out why (but avoid the all-too-easy accusation that "they didn't spend enough time on it").
Apparently this technique (along with a set of workplace practices that actively discouraged managers from trying to figure out whether their teams were actually working or doing something else) worked really well for Best Buy.